Movement of Substances Across a Plasma Membrane in Living Organisms

Diffusion in living organisms

• Gaseous exchange in the alveoli: oxygen diffuses from air in the alveolus into the blood, and carbon dioxide diffuses from the blood into the alveolus, across a thin, moist, richly capillary-supplied surface.
• Gaseous exchange in the leaf: carbon dioxide diffuses into the leaf through stomata for photosynthesis, and oxygen diffuses out, moving through air spaces in the spongy mesophyll.
• Absorption of digested food in the villus: small digested molecules such as glucose and amino acids diffuse from the intestine into the blood capillaries of the villus down a concentration gradient.

Osmosis in living organisms

Root hair cells absorb water from the soil by osmosis: the cell sap inside the root hair cell has a lower water potential than the soil water, so water moves into the cell down a water potential gradient. Guard cells also use osmosis, when water enters guard cells they become turgid and curve to open the stoma, and when water leaves they become flaccid and the stoma closes.

Active transport in living organisms

Some situations need substances to move against a concentration gradient, so the cell uses active transport instead. Root hair cells take up mineral ions, such as nitrate and magnesium ions, from the soil even when the soil concentration is lower than inside the cell.

In the nephron, useful substances such as glucose are reabsorbed from the filtrate back into the blood by active transport in the kidney tubule, even after diffusion alone would no longer favour their return.

How it is examined

Exam questions typically describe a biological structure or situation (an alveolus, a leaf, a villus, a root hair cell, a nephron) and ask you to identify which transport process is occurring and to explain it using the correct terms, such as concentration gradient, water potential, or energy from respiration. You are also expected to explain why a structure's features, such as a large surface area or thin membrane, make the transport process efficient.

All three processes in one structure: the nephron

The nephron in the kidney shows diffusion, osmosis and active transport working together in a single structure. During ultrafiltration, small molecules such as glucose, urea, water and mineral ions are pushed out of the blood into the nephron under pressure.

As the filtrate flows along the kidney tubule, useful glucose is reabsorbed entirely back into the blood by active transport against its concentration gradient, most of the water is reabsorbed by osmosis following the reabsorbed solutes, and urea, which the body no longer needs, is left to pass out in the urine. This combination allows the kidney to reclaim valuable substances while still removing waste.

Common misconceptions

Worked exam-style question

Question. Digested food molecules are absorbed at the villus lining the small intestine, which has a folded surface covered in microvilli and a dense network of blood capillaries close to its surface. (a) Name the process by which glucose usually moves from the intestine into the blood capillary.

(b) State one structural feature of the villus that speeds up this process, and explain how. (c) Explain why some nutrients occasionally need to be absorbed by active transport rather than by the process in (a).

Model answer. (a) Glucose usually moves by diffusion, down its concentration gradient from the intestine into the blood. (b) The microvilli greatly increase the surface area of the villus, allowing more glucose molecules to diffuse across at the same time, speeding up absorption.

(c) If the concentration of a nutrient becomes higher in the blood than in the intestine, it can no longer move in by diffusion down a gradient, so the cell must use active transport, spending energy from respiration, to continue absorbing it against the gradient.

Practice question

A unicellular organism such as Amoeba has no lungs, gills or other specialised gas exchange structure, yet it still exchanges oxygen and carbon dioxide efficiently with its surroundings. Suggest why a specialised structure is not necessary in this case.

Exam tip

Key terms

• Diffusion, the net movement of particles from a higher to a lower concentration.
• Osmosis, the net movement of water from a higher to a lower water potential across a partially permeable membrane.
• Active transport, movement of substances against a concentration gradient, requiring energy.
• Alveolus, the air sac in the lung where gaseous exchange occurs by diffusion.
• Villus, a finger-like projection in the small intestine that absorbs digested food.

Source:SRC-DSKP-EN

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